Basically allows you to sync your Mac (Calendar/Contacts/files/photo's) to a webspace, which you can view on a web browser on a PC or another machine - or directly back to a second or third Mac.
I'm considering the free-trial, to see whether or not it'd be useful. I'll have an iMac and Window XP laptop, which I'd like to keep in sync (contacts/calendar).
I think it's weird how (unless I am missing something) on the apple site it will not tell you the price straight away, until after you have put your card details in
EDIT: Sorry, my bad. Just noticed the prices are available in the apple store online.